Field of the Invention
The disclosure generally relates oil field equipment. More particularly, the disclosure relates to the blowout preventers.
Description of the Related Art
In gas and oil wells, it is sometimes necessary to seal against or even shear a tubular member disposed therein and seal the wellbore to prevent an explosion or other mishap from subsurface pressures. Typically, the oil field equipment performing such a function is known as a blowout preventer (“BOP”). A BOP has a body that typically is mounted above a well as equipment in a BOP stack.
FIG. 1A is a perspective schematic view of a typical prior art Type U blowout preventer (“BOP”). FIG. 1B is a top schematic view of the Type U blowout preventer of FIG. 1A. Portion “A” of FIG. 1B shows the BOP assembled, and portion “B” of FIG. 1B shows the BOP disassembled in part with a bonnet separated from the main body of the BOP. The most widely used BOPs are known as a Type U blowout preventer. A typical Type U BOP 2 has a body 4 with a through-bore 6 having a longitudinal centerline 7 through which a drill pipe or other tubular member can extend. Bonnets 8 are mounted on opposing sides of the body 4 to extend laterally from the body and enclose various hydraulic and mechanical components. A plurality of bolts 9 firmly attach the bonnets 8 to the body 4 of the BOP. Opposing ram bores 10 are formed in the body 4 and align with the opposing bonnets 8. The ram bores 10 are disposed transverse, generally perpendicular, to the longitudinal centerline 7 of the through-bore 6. Type U rams 12 are slidably disposed in the ram bores 10 to move axially along the ram bores. The rams 12 are slidably sealed with the walls of the ram bores with elastomeric ram seals disposed on the rams, having a top ram seal 14 and a side ram seal 16 on each side. The ram seals 14, 16 form a characteristic “U” shape that allows sealing along the sides with the side ram seal 16 closer to the leading edge 13 of the ram (that is, toward the through-bore 6) and sealing along the top of the ram with the top ram seal 14 distal from the leading edge 13 to allow space for blades, other seals, and various other components that would be exposed to the pressure in the through-bore 6. Actuators are disposed in the bonnets and connected at the outward ends of the rams 12 to cause the rams to move along the ram bores 10, and close around or shear drill pipe disposed in the through-bore. A hydraulic actuator 18, such as a piston, can be disposed in a chamber 20 in the bonnet 8. Hydraulic pressure exerted on an outward face 22 of the actuator forces the piston and the ram coupled to the piston to move inwardly toward the through-bore 6. A mechanical actuator 24 can be used in lieu of the hydraulic actuator. The mechanical actuator 24 can include a threaded shaft that engages a slidable shaft coupled to the ram 12, so that the ram can be screwed inwardly toward the through-bore by turning the shaft inside a housing 25 attached to the bonnet. Different types of blades can be coupled with the rams 12 depending on the style of the blowout preventer, and typically include “pipe,” blind, or shear blades.
When closed around the pipe or having sheared the pipe, depending on the type of blade, the rams 12 are pressure-energized using wellbore pressure to increase the sealing force against a tubular product in the through-bore, and help maintain the seal in case of hydraulic pressure loss on the rams. Further, mechanical locking systems, wedgelocks, and other devices can also help hold the rams closed, if the hydraulic pressure loss occurs.
However, the pressure-energized and locking aspects assume the ram seals 14 are in a suitable condition to seal under the pressure. Ram seals 14, 16 on a Type U BOP are in a constant state of compression while at any position within the ram bore. As the sealing elastomer ages, it has the potential to lose elasticity while in its compressed shape, thus reducing sealing characteristics when it may be needed, potentially years later on a wellhead.
Additionally, because the Type U BOP ram 12 seals along the entire length of the ram bore 10 with the ram seals 14, 16, disassembly is not generally performed manually due to the force required to pull the ram out of the ram bore. The disassembly and removal of the ram (and the bonnet) is done with hydraulic force. For example, a ram change shaft 26 can be inserted through a ram change chamber 30 in the bonnet 8 and threaded into the BOP body 4. An outward end of the shaft 26 can include a piston 28 sealingly engaged with the chamber 30.
To disassemble the BOP, the bolts 9 can be removed and hydraulic pressurized fluid applied to the ram that fills into the chamber 20 and is ported into ram change chamber 30. The fluid pressure fills the chamber 30 and pushes the bonnet 8 away from the main body 4 of the BOP, as shown in the portion “B” on the right side of FIG. 1B. The ram 10 has been removed from the ram bore 12 and the bonnet and components attached thereto can be removed from the body, and replaced or repaired.
However, this dependency on hydraulic pressure to disassemble the Type U BOP can lead to two challenges. There must be hydraulic pressure. If the hydraulic system is unavailable through power loss or failure, the BOP is difficult to dissemble manually. Further, the hydraulic ram change feature on Type U BOPs is expensive to manufacture.
Therefore, there remains a need for improved blowout preventer to be able to improve the sealing characteristics of the rams in the BOP for containing the wellbore pressure, and reduce the need for a hydraulically operated system to remove the bonnet for replacement or repair of the internal components of the BOP.